Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: I vowed to observe i'tikaf in al-Masjid al-Haram. When I accepted Islam, I asked the Prophet (peace and blessings be upon him) about it, and he (peace and blessings be upon him) said: "Fulfil your vow."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20099